Ethylene copolymer elastomers are polymerized from ethylene and an alpha-olefin such as propylene, 1-butene, 1-hexene, 1-octene, or the like. The comonomer disrupts ethylene crystallinity at room temperature, which would otherwise produce a stiff and inelastic polymer. Typically, ethylene copolymer elastomers comprise less than about 75 wt % ethylene so as achieve a low modulus and good elastic recovery in thermoset applications.
For elastomer applications requiring resistance to temperatures in excess of about 70° C., the ethylene copolymer elastomer must be crosslinked. Free radical crosslinking may be accomplished by compounding the elastomer with peroxide or exposing an article to high energy radiation such as an electron beam. Improved reactivity towards free radical curing can be achieved by copolymerizing an unsaturated cure site monomer with ethylene and an alpha-olefin. The unsaturated cure site monomer also permits curing of the ethylene copolymer elastomer by sulfur, phenolic resin, or hydrosilation.
Ethylene copolymer elastomers comprise ethylene and an alpha-olefin, with or without a non-conjugated diene cure site monomer. Ethylene copolymer elastomers comprising ethylene, propylene, and a non-conjugated diene monomer such as ethylidene norbornene are widely available and known as EPDM rubber. In the uncrosslinked state, these polymers are generally referred to as ethylene copolymer elastomer gums, or ethylene copolymer elastomer gum rubbers. Examples of commercially available crosslinkable ethylene copolymer elastomer gums include Engage® and Nordel® from The Dow Chemical Company, Midland, Mich. USA and Vistalon® and Exact® from Exxon-Mobil Corp, Irving, Tex., USA. The resulting crosslinked articles have good heat and water resistance as well as desirable electrical properties, making them suitable for wire and cable jacketing and a wide range of automotive applications including hoses, ignition cable jacketing and boots, vibration dampeners, weather stripping, and seals.
Resistance to heat aging is a particularly desirable property in rubber parts that are used in certain wire and cable jacketing applications, as well as many under the hood automotive applications, e.g. hoses, gaskets, and seals. Because such parts may be exposed to temperatures in excess of 150° C. for periods of time, including up to several hours on a regular basis, degradation of physical properties through oxidative embrittlement can occur. In ethylene copolymer elastomers, a reduction in the strength and extensibility of the crosslinked article often results. Such effects are disclosed for example in the published presentation “A New Grade of EPDM with Improved Processing Characteristics for Automotive Hose” by M. Welker et al., presented at the ACS Rubber Division technical meeting, October 2011. Methods to enhance heat aging resistance of crosslinked ethylene copolymer elastomer compounds have involved increasing ethylene content and decreasing carbon black content in the compound to maintain constant hardness, but the high ethylene level increases polymer crystallinity and degrades elastic properties. More effective antioxidants have also been sought. However, there is still a need to improve the high temperature resistance of crosslinked articles from ethylene copolymer elastomer compounds.
Ethylene copolymer elastomer compounds generally comprise both reinforcing filler and plasticizer. Reinforcing filler increases hardness and strength of the cured compound, whereas plasticizers lower the viscosity of the compound, as well as the hardness and strength of the cured article. Manipulating filler and plasticizer levels in a curable ethylene elastomer compound allows the cured articles to meet a variety of application requirements, but hot air aging can be modified only slightly through these techniques.
It has now been found possible to produce cured ethylene elastomer compositions having excellent hot air heat aging resistance through the use of a small amount, relative to other components in the composition, of polyamide dispersed in an ethylene copolymer elastomer gum. The polyamide content is sufficiently low that compound hardness, strength, elasticity, and other properties such as processability are essentially unchanged as a result of inclusion of the polyamide.
U.S. Pat. No. 3,965,055 discloses vulcanizates prepared from a blend of rubber and 2% to 10% of a crystalline fiber-forming thermoplastic, wherein the thermoplastic is dispersed in the rubber component in particles not greater than 0.5 micron in cross section with a length to diameter ratio greater than 2. The fibrous thermoplastic increases the green strength of the compound, thereby reducing the tendency to sponge during low pressure curing. The rubber may be EPDM and the thermoplastic may be a polyamide.
U.S. Pat. No. 4,966,940 discloses vulcanized rubber compositions comprising an ethylene alpha-olefin copolymer rubber, an ethylene alpha-olefin copolymer rubber containing an unsaturated carboxylic acid or a derivative thereof, and 5 to 100 phr of a polyamide resin.
U.S. Pat. No. 6,133,375 discloses blends of functionalized rubbers with thermoplastics in which the thermoplastic component is dispersed in the rubber phase. Following addition of a curative for the rubber, the composition is crosslinked to produce a vulcanized article. Examples of functionalized rubbers disclosed include acrylic rubbers such as nitrile-butadiene rubber, hydrogenated nitrile-butadiene rubber, epichlorohydrin rubber, and rubbers on which reactive groups have been grafted, such as carboxylated nitrile-butadiene rubber. Non-functionalized rubbers include EPDM, and these may be used provided a functionalized rubber is present. Thermoplastics that are disclosed include polyetherester block copolymers, polyurethanes, polyamides, polyamide ether or ester block copolymers, and mixtures of polyamides and polyolefins. The thermoplastic component is present in sufficient amounts to increase the modulus at small elongations, and reduce the breaking stress by no more than 10% in the vulcanized article, relative to a compound lacking the thermoplastic component.
U.S. Pat. No. 8,142,316 discloses power transmission belts comprising an elastomeric/thermoplastic material for the insulation section of the belt. The thermoplastic may be a polyamide and the elastomer may be an EPDM rubber. The thermoplastic is present in amounts of 10 to 50 phr, and the rubber is present in amounts of 50 to 90 phr. The elastomeric/thermoplastic material further contains 15 to 75 phr carbon black.
U.S. Pat. No. 7,608,216 and U.S. Patent Application Publication 2006/0100368 disclose compositions prepared by admixing an uncured elastomer with a thermoplastic polymer or another uncured (gum) elastomer. Techniques such as fractional curing, partial dynamic vulcanization, or the use of high performance reinforcing fillers are disclosed to increase the green strength of the uncured or partially cured compound. The admixed compositions may be subsequently crosslinked with a curing agent for the elastomer component.
Thermoplastic elastomers comprising polyamides and ethylene copolymers are also known. Such compositions, often known as thermoplastic vulcanizates or TPVs, are produced by crosslinking an elastomer while simultaneously mixing with molten thermoplastic. This process, known as dynamic vulcanization, causes the thermoplastic to become the continuous phase of the blend. EP922732 discloses TPVs comprising a thermoplastic that may be a polyamide, a carboxylic acid, epoxy, hydroxyl, anhydride, or amine functionalized rubbery ethylene, and a halogenated rubbery of para-alkylstyrene and monoisoolefin of 4 to 7 carbon atoms.
Polymer 43 (2002) 937-945 discloses blends of EPDM and polyamides compatibilized by maleic anhydride, glycidyl methacrylate grafted EPDM, or chlorinated polyethylene (CPE). The polyamides have a melting peak temperature of 150° C. or less. The cured compositions comprise 46 to 50 phr N220 carbon black. The polyamide is present at high levels (at least 10 wt %), in the form of a fibril.
Polymers & Polymer Composites 11(2003) 179-188 discloses compatibilized blends of EPDM and low melting peak temperature polyamide (150° C.) of at least 10 phr or more. The blends are cured at 160° C., above the melting peak temperature of the polyamide, and display weak cure response (MDR torque increase of 2 dN-m or less). The authors note that it is difficult to blend EPDM with high melting peak temperature polyamides such as PA6 or 6/6.
It has now surprisingly been found that when a low level of polyamide particles with a high melting peak temperature are dispersed in an ethylene copolymer elastomer compound, the resultant cured compositions exhibit enhanced resistance to physical property loss during heat aging. The polyamide content is sufficiently low so as not to interfere with the tensile strength, modulus, and hardness, or elastic properties that characterize conventional ethylene copolymer elastomer compositions.